temperatures had to remain above 1 billion degrees in order for atoms to form. b. nebulae condensed and heated to become spinning accretionary disks, then protostars, then stars. c. the process of atom creation, termed Big Bang nucleosynthesis, produced all known natural elements. d. the Earth formed 4.5 billion years ago by the accretion of several small planetesimals.

The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.